I can undrstand why Ferrari wouldn't be that concerned given the history of that particular engine and i think it would be quite realistic to expect trouble with it when reused.
As for overheating it's difficult to say but i doubt that conditions seen in these early few races (Bharain / Malaysia) will be encountered again during the course of the season but it was noticable they created extra "gill" vent slots for those races.
Having said that there may well be an arguement that the packaging may be the cause of the problem but not in the conventional sense of just squeezing the bodywork on to tightly. I guess what needs to be remembered is that this engine is more than capable of reliable operabilty proven in previous years given that thr revs are now limited to 18k and last year it also coped with the stresses of the KERS system.
What is different this year is related to packaging but has far more to do with the angle at which the engine / gearbox is mounted, at 3.5 deg angle. That could well have a bearing on the problems as different stresses will undoubtly be encountered and i'd imagine would neccesitate some different solutions to the norm around the areas of heat dissipation and fluid dynamics (fuel/oil/coolant/hydraulics etc).
Maybe this reliabilty problem is an effect of chasing aero?